The geochemical signature of a felsic injection complex in the continental crust: Opinaca Subprovince, Quebec

Abstract

The Archean Opinaca Subprovince of the Superior Province in northern Québec is an injection complex consisting of lower granulite facies metasedimentary terrane that was pervasively injected with a large volume of leucogranitic melt which forms innumerable thin dykes and veins.The typical microstructure of most leucogranites can be divided into two groups, 1) one has an open framework constructed of K-feldspar crystals in which the interstices are filled with albite-rich plagioclase and quartz and, 2) a microstructure defined by a framework of albite-rich plagioclase crystals in which the interstices are filled by quartz and, in some cases, small pink garnets. However, pegmatitic and aplitic textures also occur.The leucogranites that form the injection complex are all SiO2-rich (above 70 wt.%) and have low (FeOt+MgO+TiO2) contents compared to granitic melts obtained from partial melting experiments. The mafic minerals were fractionated from the magma before its emplacement into the injection complex as leucogranite. The leucogranites form a coherent trend on major element diagrams that is best explained by the removal of a cumulate dominated by K-feldspar from a granitic “initial melt” composition. The cumulate rocks are enriched in K2O, whereas the residual melts are richer in SiO2 and Na2O.Based on these results, leucogranites with a K-feldspar framework are interpreted as cumulate rocks. This indicates that the early-crystallising calcic plagioclase had already been fractionated from the magma before its injection into the complex. The leucogranites have rare earth element (REE) patterns typical of feldspar accumulation. The residual melts extracted after the K-feldspar framework developed are those that have the framework of albite-rich plagioclase. These have REE patterns typical of highly evolved melts from which accessory minerals have also been fractionated.The highly evolved nature of the leucogranites that were injected into granulite facies host rocks in the Opinaca Subprovince has several important implications for crustal evolution. 1) The large volume of evolved melts emplaced at depth released a significant amount of H2O into the adjacent crust, thus injection complexes should be considered as a potentially important source of H2O in the middle crust that is capable of rehydrating the granulite terranes into which they are injected. 2) A corollary of the evolved nature of the leucogranite in the injection complex is that there should be evidence of accumulation of the early fractionated material still deeper in the crust. 3) These findings are natural examples that corroborate predictions from numerical modelling which suggest that the whole process from melting to crystallisation of leucogranite can be confined to deep levels of the crust, below, or close to, the solidus. This has important impact on our general view of the transfer of leucogranites in the continental crust which is the main mechanism of crustal differentiation.